High-voltage, low-background electronic camera

ABSTRACT

Electron cameras or image tubes employ a photoemissive semitransparent cathode upon which light may be focused to emit electrons which then are focused by electron optics either directly upon an electron-sensitive recording material or upon a phospher which provides recorded data for subsequent analysis. The high sensitivity of these cameras renders them particularly useful for direct astronomical studies although the problem of high-background interference due to field emission and other related phenomena have imposed serious limitations upon existing image tubes. The present method permits operation of these tubes at voltages up to external breakdown potentials by providing a tube-fabrication process that eliminates sharp points and assures cleanliness of the electrodes. Undesirable field emissions thus are minimized. The electrodes or electronic lens members are formed initially by machine-working relatively massive tubular metal parts into the final desired tubular shape of the electronic lens members. The wall thickness of these massive parts must be greater than the maximum thickness of the tubular walls of the final shape of the members. When formed, the lens shapes are polished and mounted with the photocathode in operative disposition. To assure the cleanliness, a vacuum is created and a source of electron-emissive material then moved through the vacuum into close proximity with the mounted photocathode. The source is heated to promote a vapor-transfer directly onto the photocathode. Selective deposition of the source material is assured by heating the electronic lens members while maintaining the cathode in a relatively cool condition.

United States Patent [72} inventor Gerald E. Kron Flagstaff, Ariz.

[21] Appl. No. 887,233

[22] Filed Dec. 22, 1969 [45] Patented [73] Assignee Sept. 14, 197 1 The United States of America as represented by the Secretary of the Navy [54] HIGH-VOLTAGE, LOW-BACKGROUND Primary Examiner-.lohn F. Campbell Assistant Examiner-Richard Bernard Lazarus Att0rneys--R. S. Sciascia and Paul N. Critchlow SEE no.4

ABSTRACT: Electron cameras or image tubes employ a photoemissive semitransparent cathode upon which light may be focused to emit electrons which then are focused by electron optics either directly upon an electron-sensitive recording material or upon a phospher which provides recorded data for subsequent analysis. The high sensitivity of these cameras renders them particularly useful for direct astronomical studies although the problem of high-background interference due to field emission and other related phenomena have imposed serious limitations upon existing image tubes. The present method permits operation of these tubes at voltages up to external breakdown potentials by providing a tube-fabrication process that eliminates sharp points and assures cleanliness of the electrodes. Undesirable field emissions thus are minimized. The electrodes or electronic lens members are formed initially by machine-working relatively massive tubular metal parts into the final desired tubular shape of the electronic lens members. The wall thickness of these massive parts must be greater than the maximum thickness of the tubular walls of the final shape of the members. When formed, the lens shapes are polished and mounted with the photocathode in operative disposition. To assure the cleanliness, a vacuum is created and a source of electron-emissive material then moved through the vacuum into close proximity with the mounted photocathode. The source is heated to promote a vapor-transfer directly onto the photocathode. Selective deposition of the source material is assured by heating the electronic lens members while maintaining the cathode in a relatively cool condition.

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PATENTEUSEPMIQ?! $604776 sum 1 or 2 INVENTOR GERALD [5 KRON v. ail U.

PATENTED SEP 1 41971 I sum 2 or 2 a aw\\\\\\\ HIGH-VOLTAGE, LOW-BACKGROUND ELECTRONIC CAMERA The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION The present invention relates to electronic-image tube cameras and, in particular, to processes for making the photocathode and electronic lens members of these cameras.

The limitations of present day cameras of the type under consideration are, as already indicated, due in significant part to the manner in which the focusing electrodes as well as the photocathodes are formed. For example, the well-known Lallemand electronic camera requires a complete high-vacuum preparation of the photocathode each time the tube is used since, in the method of Lallemand, the photocathode is sacrificed each time new plates are loaded into the camera. Another camera described in the US. Pat. No. 3,266,505 issued Aug. 16, l966 to the present inventor solves this difficulty by providing the camera with a special valve capable of isolating the photocathode section of the camera from its plateholder section so as to permit removal and replacementof the plate while maintaining the vacuum of the photocathode section. This cross-referenced patent will be described in considerable detail since its structure is pertinent to the improvement of the present invention. I

The present improvements are more directly concerned with other deficiencies such, for example, as the difficulty experienced by the prior art in vapor-depositing photoemissive material directly onto a photocathode mount which is in operative position in the camera. In operative position the photocathode is mounted close to the electrodes so that any procedure for transferring the photoemissive material onto the cathode must provide some manner of preventing the deposition of this material onto the other electrodes. If the electrodes are coated with any appreciable amount of this material, the background effects caused by cold-electron emission materially degrades the recorded image. To avoid this difficulty certain techniques, such as the use of electrode shields or screens, have been suggested. However, such shields are extraneous elements which themselves can produce undesirable background effects. Also, they must be movably mounted within the evacuated camera so as to permit their being placed in front of the electrodes or lens during deposition and then removed to another part of the camera to permit effective operation of these lens members. Other fabricating techniques prefer the alternative of preparing the cathode externally of the camera and then mounting it in operative position after it is coated with its photoemissive material. Obviously, such external preparation creates its own problems and difficulties.

Effective operation of these cameras also is reduced by the creation of relatively high electrostatic fields due to relatively sharp points or projections on its operative components such as the electronic lens members. For example, the more conventional manner of forming these lens members is to shape them directly from sheet metal parts which are bent or otherwise formed into the desired lens configurations. The difficulty arises because, regardless of the degree to which the parts are polished, they still present relatively sharp points which, when subjected to high voltage operation of the camera produce the undesirable field emissions that interfere with sensitivity.

One object of the present invention is to provide a method of fabricating the operative components of the camera so as to eliminate to the maximum degree possible the creation of field emissions due to relatively sharp points or protuberances.

Another related object is to provide a method of coating the photoelectrode of the camera in situ, this latter method being characterized by simplicity and its ability to maintain the cleanliness of the electronic lens membersdurin g the coating.

These and other objects of the invention are achieved generally by fabricating the electronic lens components of the camera from massive metal parts as contrasted with the sheet metal of the prior art. The formed parts then are mounted in operative disposition relative to the photocathode and these parts are selectively heated during the deposition of the photoemissive material on the photocathode. The photocathode is maintained relatively cool to promote a selective transfer of photoemissive material. Other more specific fabrication steps of the present invention will be described with reference to the valve-controlled camera described in the cross-referenced patent.

BRIEF DESCRIPTION OFTHE DRAWINGS A preferred embodiment of the invention is illustrated in the accompanying drawings of which FIG. 1 shows the socalled valve-control camera in elevation, the upper portion of the camera being sectioned to illustrate underlying details of the operative components contained within its casing;

FIG. 2 is a partial face view of the vacuum valve of the camera with certain parts broken away to show underlying parts;

FIG. 3 illustrates a method of vapor-depositing photoemissive material onto the photocathode of the valve-control camera; and

FIG. 4 is an enlarged detail of the circled portion of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the camera or image tube there shown is formed of three individual and separate sections. These being a first photocathode section 1 which terminates with a tube base member 2, a vacuum-valve section 3 and a plateholder assembly 4 removably secured to vacuum-valve assembly 3 by a coupling 6. Vacuum valve section 3 is fully disclosed in US. Pat. No. 3,266,505 to which reference can be made for such additional detail as may be desired. The present description will consider this vacuum valve section primarily from a functional viewpoint although its major components will be identified.

The camera illustratedin FIG. 1 is somewhat conventional to the extent that it includes a photocathode 7 mounted in the forward end of section 1 to receive radiation from a telescope 8, a portion of which is shown in the dotted lines of the figure. Electrons emitted by photocathode 7 are focused by electron lens members 9, ll, 12 and 13 directly onto a plate 15 which, by way of example, can be a commercially available, nuclear track plate such as the Ilford nuclear track plate. A phosphor coating may be employed to provide a record for subsequent reproduction by photography. The photoemissive cathode, as well as the entire interior path of travel of the emitted electrons, is maintained in a region of very high vacuum and this path, for purposes of description, will be referred to as bore 16.

To provide the necessary vacuum, section 3 mounts a conventional vacuum pump which communicates with bore 16 through another bore opening 17. Also, section 3 mounts a pair of ion pumps, being in communication with bore 16 through a port 18 (FIG. 2) while the other communicates with the main bore through a port 20. The ion pump for port 18 may be used to produce the final desired high vacuum for the camera after general evacuation has been achieved by use of the conventional vacuum pump. The other ion pump is provided to draw off leakage when the valve of section 3 is closed and the plate chamber is opened to air and, for this purpose, it communicates with a leakage chamber 21 shown in FlG. l.

The valve arrangement of section 3 is described in detail in the referenced patent. For present purposes, it simply can be noted that it includes a valve gate or coin 22 adapted to slidably open and close bore 16 of the camera and that this gate is so disposed relative to the ports of the vacuum system as to permit the entire bore to be evacuated when the gate is open while permitting the plateholder section to be evacuated when the valve is closed. Also, when the valve is closed, the ion pump coupled to port 20 is used as a leakage pump to maintain the desired high vacuum in photocathode section 1. Valve gate 22 normally is free to slidably move in a counterbore 23 formed in the valve body and the slidable movement can be achieved by tilting the entire camera in one direction or another. However, when it is desired to maintain valve gate 22 in its closed position, jackscrews 24 can be turned to move a seat against the valve to tightly clamp it in the desired disposition.

The structure of plateholder section 4 also is unique, although its structure is not intended to form a part of the present invention. As previously stated, section 4 mounts image plate upon which the image of the radiations received by photocathode 7 are recorded. In front of the plate 15 is a shutter 26 which can be a butterfly structure having wing members (not shown) one of which can be placed over image plate 15 directly in front of bore 16 of the camera so as to prevent exposure of the plate. Also, as may be noted in FIG. 1, bore 16 aligns with only a relatively small portion of plate 15 which, in turn, is a relatively large circular plate. This arrangement is appropriate since it permits plate 15 to be rotated circumferentially to expose successive sections in the plate. Thus, the plate may be used for a plurality of exposures before it is necessary to replace it.

A particularly appropriate manner of rotating plate 15 is provided by a magnetic plate advance tool 27 which, in essence, is a magnet adapted to be rotatably moved by a knurled handle 28. Rotation of the magnet produces a following movement of an iron core member 29 to which it splined a rotor 31 having a pinion 32 meshed with a gear 33 which, in turn, is splined to a hub 34 of a turntable 35. Plate 15 is mounted on turntable 35 so that it can be rotated manually by means of knob 28 of the plate advance tool. Most suitably, the shutter can be indexed into an open and closed position and, although not shown in the drawings, the arrangement may include projecting pegs or pins to engage and move the shutter into one position or another. These pins may be carried by the shutter in appropriate positions to be engaged by sections of the rotating plate assembly so as to permit movement of the shutter. A plurality of pins may be employed so that rotation in one direction. opens the shutter while rotation in the other causes it to close. Other conventional shutter opening and closing arrangements obviously could be substituted.

The principal features of the present invention are more directly concerned with the manner or method of forming the components of photocathode section 1. Some of the operative components which already have been identified include photocathode 7 which receives the radiations from telescope 8 and which emits electrons that proceed along bore 16 to impinge upon plate 15. Also, electron lens components 9, ll, 12 and 13 focus the emitted electrons as well as accelerate them by means of the electric potential applied to these electrodes. A voltage source 37 is applied to the electrodes through conduit 38 which connects to section 1. Other portions of the bore provide the anode for the arrangement. All of these components are in a high vacuum environment during operation and consequently are disposed in an airtight casing communicated with the vacuum pumps of section 3. At its forward end, the casing is formed by photocathode mount 7 which may be sapphire member or other conventional types, the photocathode amount being brazed to kovar which, in turn, is sealed to electron lens member 9. Centrally disposed electrodes 11 and 12 may be formed of one member although, according to present procedures, it is preferable to provide two members coupled together by a pair of coupling rings 41 and 42 that clamp about annular flanges provided on the electrodes and are held in their clamped position by a plurality of screws 43. To assure the seal the rings clamp about a gasket disposed between their meeting surfaces and such a gasket may be formed of thin-annealed pure gold wire squeezed to trodes. The rear of the airtight casing is formed by tube base 2 which, as seen, includes electrode 13. Surrounding the entire airtight casing there is a magnetic shield 48 formed, as shown,

of a cylindrical metal ring and of front and back shield plates. Directly beneath the shield is a silicone rubber insulator member 49.

Although the arrangement just described is a conventional one, the electrodes, as well as photocathode 7, are formed in a special manner which has been found to permit high-voltage operation of the tube without resulting in a degrading of the tube image due to background interference which, as is known, produces a general blackening of the f eld created by unwanted electrons coming from the tube itself. Such background has been found to be due to several sources such as light caused by corona and related phenomena, field emission of particles inside the tube and particles released from ionization from the residual gas inside the tube. Although certain of these phenomena cannot be fully avoided, it has been discovered that the background level can be very significantly minimized by forming electrodes 9, 11, 12 and 13 in special manners and also by maintaining the cleanliness of these electrodes by assuring that the photoemissive material deposited on photocathode 7 does not deposit on any of the other electrodes.

As to the formation of electrodes 9-13, it first might be noted that in conventional practice such electrodes are formed from sheet metal which has the initial desired thickness of the electrodes and which then is shaped into the desired configuration. Although such sheet metal electrodes have certain advantages such as the facility with which they can be out-gassed, they nevertheless have been found to possess series disadvantages in that they present relatively sharp uneven or rough surfaces which when subjected to the high voltage at which it is desirable to operate the tube produce substantial field emissions which provide a portion of the unwanted background. High voltage operation of these tubes is most desirable since it permits a clearly defined image of the radiation to be formed on the image plate in a minimum of exposure time. In fact, the prior art cameras presently used in astronomical observations recognize the desirability of operating at high voltages although, due in part to the manner in which the electrodes are made, these cameras have been unable to utilize the high voltage at least in what might be called a single stage tube. Instead, some of the prior art image tubes utilize a two-stage operation in which the electrons are accelerated in separate stages each at about 10 kilovolts. Use of the desirable higher voltage of about 30 kilovolts has been found to be impractical since the resulting photograph is so obscured as to be of little or no use.

The present invention contemplates making electrodes 9-13 from massive metal parts as opposed to fabrication of these electrodes from the sheet metal used in prior cameras. As shown in FIG. 1, the configuration of all of these electrodes is tubular with electrode 9 being dish-shaped with an interior curvature true to the spherical surface of photocathode 7. Electrodes l1 and 12 are conical rings or plates and it is to be specially noted that the terminal ends of each of these electrodes is materially thickened to at least twice the width of the plate, these thickened terminal ends being identified by nu- I merals 51 and 52. Electrode 13 of tube base 2 also is tubular and has its terminal end 53 thickened in a manner similar to the central electrodes.-ln greater particular, it will be noted that electrodes 11 and 12, which may be formed of CR8, are slightly different in configuration to the extent that electrode 11 has an angle of inclination which is less than that of electrode 12 and, further, electrode 11 is shorter than electrode 12. The angle of inclination of electrode 11 as contemplated for the present camera is 1 115 while that of electrode 12 is 2745. The thickened ends of these electrodes are held to very close tolerances, terminal end 51 having an 0.93 radius, while terminal end 52 preferably has a 0.125 radius. The formation of tube base 2 with its electrode terminal end 53 also is quite critical at least to the extent that it is mandatory that all sharp comers of this member be completely removed. Terminal end 53 may have a 0.50 radius and it further should be noted that portions 54 and 55 of this member also are rounded to close tolerances most suitably with a radius of 0.3 1. It will nevertheless be appreciated that the actual dimensions and curvatures of these members may vary somewhat with the camera in which they are incorporated and that the present dimensions are given principally by way of example.

Again, the principal feature insofar as formation of these electrodes is concerned resides in the fact that they are formed from massive metal parts so as to permit maximum smoothness and rounded contours. By the term massive metal parts it is meant that each of these electrodes is formed from a solid piece of metal of substantially greater original thickness than the maximum width of the thickest portion of the tubular wall of the finally formed electrode shape. Although the electrode can be formed by machining solid cylinders of CR8 metal, it is preferred to start with tubular parts. However, each of the tubular parts must, as stated, be thicker than the maximum thickness of the electrode to be formed. Thus, with reference to electrode 12, for example, the metal part from which it is machine-fon'ned would be a tubular part having a wall thickness greater than the distance between the minimum internal diameter of the finally formed conical shape and the maximum external diameter of this shape. Stated in another manner, this wall thickness must be greater than the divergence distance of the cone. Similar situations would exist with regard to the formation of electrodes 9 and 11, it being noted that the metal part from which electrode 9 is to be formed must be of quite a substantial wall thickness since it must be greater than the distance between the photocathode and the glass insulating member 44 to which it is secured. Tube base 2 also is formed in this manner and here again the initial wall thickness must be substantial to permil the final form or shape of the electrode to be machined from the tubular body of the initial metal part. By the term machining of these metal parts, it, of course, is meant that the metal parts are placed on a lathe or other metalworking machine and carefully reduced in size and shape to the final curvatures desired for the electrodes. In this work, close tolerances to achieve particularly the rounded contours of the end portions is mandatory. In practice, it has been found that electrodes formed in this manner are significantly smoother, particularly after having their machine surfaces highly polished, than the corresponding sheet metal electrodes of the prior art cameras.

With the electrodes so formed, the final steps in the fabrication of section 1 of the camera than can proceed. in describing the fabrication, it will be assumed that these electrodes, as well as all other parts of the camera, have been thoroughly out-gassed by baking and cooling these components in vacuum. This procedure is a conventional one used in the fabrication of all cam a tubes of this type.

The principal fabrication step aside from the formation of the electrodes from the massive metal parts is concerned principally with maintaining the cleanliness of the electrodes during deposition of a photoemissive material, such as antimony and cesium, on photocathode mount 7. If these materials deposit on the electrodes or electron lens components 9-13, subsequent operation of the tube at the desired high-voltage input very readily causes the emission of unwanted electrons to increase the background level beyond that permissible for acceptable photography. Prior art procedures also have been concerned with the protection of the electron lens components during deposition of this photoemissive material on the photocathode, but the techniques employed principally have provided screen members to physically protect the electrodes during the deposition. Obviously, screen members are extraneous and introduce undesirable complications particuiarly since they must be moved into and out of screening disposition while in the high-vacuum tube.

The present invention maintains cleanliness by selectively heating the electrode during deposition of the photocathode material. Referring to FIG. 3, it will be noted that, after electrodes 9-13 have been assembled in the configuration shown in FIG. 2, the airtight casing then is wrapped with strip heaters 57, the strip heaters being applied only to those portions of the casing surrounding or approximate to the electrodes so that the area surrounding photocathode 17 can be maintained at a relatively lower temperature. in particular, two strip heaters may be utilized, these heaters being 400 watt each operated at volts AC to maintain a temperature during processing of the cathode of about C. while the photocathode remains substantially at room temperature.

Deposition of the photoemissive material may be accomplished by use of a special tool generally designated by numeral 58 and illustrated in FIG. 3. The tool, which is shown rather schematically, includes a glass tube 59 having one of its ends closed and the other provided with a coupling member 61 capable of being sealably coupled to vacuum valve section 3 of the camera when plateholder section 4 is removed. Coupling 61 may be in any form and, if desired, a special adapter 62 can be provided for attachment to section 3 to receive the coupling. Within glass tube 59 is a loosely mounted, slidable rod 63 having an RF pickup coil 64 at one of its ends. The other end is formed in the manner shown in FIG. 4 to provide a cap 66 on which is mounted a wire 67 to support a beat of antimony or other photoemissive substrate material such as is to be applied to photocathode mount 7. Wire 67 is electrically coupled to the RF pickup coil so that, when the coil is energized, the antimony head is heated to vaporize the metal and cause it to deposit on proximate surfaces such as photocathode. Cap 66 aids in directing the vapor onto the photocathode. Usually, image tubes of the type under consideration also use a photocathode coated with an additional photoemissive metal such as cesium and it is particularly important that the cesium be deposited only on the photocathode as opposed to other portions of the tube. To permit deposition of the cesium through tool 58, the tool is provided near its forward end with a glass tube extension 68 coupled to a cesium supply. Deposition of the cesium is achieved by heating extension 68 to cause the cesium to vaporize. Normally it is in a solid state.

Use of tool 58 includes several preliminary operations. First, the tool is sealably coupled to section 3. The main vacuum pump of section 3 then is driven to evacuate the interior and, following this, the ion pump completes the evacuation. At this stage, tube 63 is magnetically moved through the valve into proximity with the photocathode to pennit the selective deposition of photocathode materials.

The camera, with the electrodes formed in the described manner from massive metal parts and with the photocathode formed in situ by selective deposition with controlled heating, achieves several distinct advantages. First, it is easier and more convenient to operate and, because it is made almost entirely of metal, it is more rugged, smaller and easier to handle. Since the photocathode is made in situ it can be larger in size. Also, because the photocathode can be used many times, it becomes possible to calibrate it for color sensitivity over its entire area. Further, camera operation is possible at a high voltage, up to breakdown voltage, and low background is achieved in spite of the high voltage operation. Consequently the prior art necessity for multistage, low voltage operation is avoided.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

lclaim:

I. For use in the formation of an electrnic image tube camera of a type having a first section containing a photocathode mount and electronic lens members for focusa ing and accelerating images received by the photocathode, a

removable image plateholder section and a vacuum valve section coupling the first section to said removable plateholder section, said vacuum valve section being capable when the valve is open of reducing the interior atmospheric pressure of the camera assembly and when the valve is closed of reducing the interior pressure of the removable section'while maintain-' ing said reduced y nure in said first section, and said electronic lens members of the first section being in the form of tubular plates provided with thickened terminal edges having smoothly rounded surfaces;

a method of forming said first section of the camera to permit high-voltage operation within minimum degradation of the recorded image, said method including the steps of:

producing the lens members by machine-working tubular metal parts of greater wall thickness than the maximum thickness of the tubular walls of the finished lens members to reduce said parts to the final tubular shapes,

polishing said shapes to provide smooth surface lens members capable of minimizing the creation of high electrostatic fields,

mounting said lens members and said photocathode mount in operative disposition in said first section of the tube,

evacuating the tube,

maintaining said evacuated condition while moving a source of electron-emissive material into close proximity with said photocathode mount, and

heating said source to vapor-deposit said material directly onto said photocathode mount for forming the photocathode.

2. The method of claim 1 wherein said electron lens members are in the form of conically tubular plates and said massive metal parts for forming said members each having an initial wall thickness greater than the divergence distance of the walls of the lens member to be formed.

3. The method of claim 1 further including the steps of:

heating said electron lens members during deposition of said material while maintaining said cathode member at a temperature not appreciably greater than room temperature whereby said material selectively deposits on said photocathode, said applied heat being sufficient to minimize deposition of said material on the electronic lens members.

4. The method of claim 1 wherein the steps of depositing said electron-emissive material on the photocathode include:

moved into proximity with said photocathode mount.

6. The method of claim 4 further including the step of:

heating said electron-lens members during deposition of said material while maintaining said cathode mount member at a temperature not appreciably greater than minimum deposition whereby said material selectively deposits on said photocathode mount, said applied heat being sufficient to minimize deposition of said material on the electronic lens. 

1. For use in the formation of an electrnic image tube camera of a type having a first section containing a photocathode mount and electronic lens members for focusing and accelerating images received by the photocathode, a removable image plateholder section and a vacuum valve section coupling the first section to said removable plateholder section, said vacuum valve section being capable when the valve is open of reducing the interior atmospheric pressure of the camera assembly and when the valve is closed of reducing the interior pressure of the removable section while maintaining said reduced preSsure in said first section, and said electronic lens members of the first section being in the form of tubular plates provided with thickened terminal edges having smoothly rounded surfaces; a method of forming said first section of the camera to permit high-voltage operation within minimum degradation of the recorded image, said method including the steps of: producing the lens members by machine-working tubular metal parts of greater wall thickness than the maximum thickness of the tubular walls of the finished lens members to reduce said parts to the final tubular shapes, polishing said shapes to provide smooth surface lens members capable of minimizing the creation of high electrostatic fields, mounting said lens members and said photocathode mount in operative disposition in said first section of the tube, evacuating the tube, maintaining said evacuated condition while moving a source of electron-emissive material into close proximity with said photocathode mount, and heating said source to vapor-deposit said material directly onto said photocathode mount for forming the photocathode.
 2. The method of claim 1 wherein said electron lens members are in the form of conically tubular plates and said massive metal parts for forming said members each having an initial wall thickness greater than the divergence distance of the walls of the lens member to be formed.
 3. The method of claim 1 further including the steps of: heating said electron lens members during deposition of said material while maintaining said cathode member at a temperature not appreciably greater than room temperature whereby said material selectively deposits on said photocathode, said applied heat being sufficient to minimize deposition of said material on the electronic lens members.
 4. The method of claim 1 wherein the steps of depositing said electron-emissive material on the photocathode include: closing the valve of said vacuum-valve section to maintain the reduced pressure in said first photocathode section, sealably coupling a closed elongate tube to said valve section in lieu of said member plateholder section, evacuating the tube portion of said tool, opening the valve of said valve section, slidably moving a tube-encased probe portion of said tool through said open valve into proximity with said photocathode, and heating said tool to promote vapor-transfer of said source material from said probe onto said proximate photocathode.
 5. The method of claim 4 wherein said probe is magnetically moved into proximity with said photocathode mount.
 6. The method of claim 4 further including the step of: heating said electron-lens members during deposition of said material while maintaining said cathode mount member at a temperature not appreciably greater than minimum deposition whereby said material selectively deposits on said photocathode mount, said applied heat being sufficient to minimize deposition of said material on the electronic lens. 